DENVER — Inmates in state prisons can’t be held in solitary confinement for more than 15 days, the Colorado Department of Corrections announced on Thursday in the latest effort to overhaul a practice criticized as “torture” by the agency’s chief.

The changes also require that inmates who are held in solitary confinement at the discretion of prison officials get at least four hours per day outside a cell for recreation or group classes.

Colorado officials in 2011 began efforts to cut the number of people held in solitary confinement. Former state corrections director Tom Clements also tried to make it easier for people once held there to re-enter society and closed a new prison built specifically to house solitary-confinement prisoners, the still-vacant Colorado State Penitentiary II in Canon City in southern Colorado.

Further changes followed Clements’ death in 2013 after he was shot by Evan Ebel, a former inmate who had spent much of his eight years’ imprisonment in solitary confinement.

The issue is a deeply personal one for Democratic Gov. John Hickenlooper, who was close friends with Clements. Ebel’s father, attorney Jack Ebel, had testified two years before the slaying that solitary confinement was destroying his son’s psyche. And Hickenlooper mentioned the case to Clements as an example of why the prison system needed reform before offering Clements the job.

Rick Raemisch, who replaced Clements as the state prisons chief, continued his predecessor’s work on solitary confinement. In 2014, Raemisch received national attention for spending 20 hours inside a solitary cell and later wrote that the experience inspired his push to eliminate or at least reduce the practice.

In an op-ed published online Thursday by The New York Times, Raemisch wrote that prisons’ reliance on solitary confinement to punish inmates for a variety of offenses “created consequences we didn’t foresee,” including Clements’ death.

Using isolation as a punishment, he wrote “has not solved any problems; at best it has maintained them.”

A department spokesman said Raemisch wasn’t available for an interview on Thursday and wouldn’t be for some time. Raemisch didn’t appear to speak with any Colorado media about the change.

Advocacy groups have long criticized the effect of solitary confinement on inmates. Other states have made sweeping changes in recent years, including preventing its use on juveniles and people with serious mental illness.

Raemisch described the changes announced this week as “unique to Colorado.” New Jersey lawmakers in 2016 approved a bill restricting the use of solitary confinement to 15 consecutive days but the state’s Republican Gov. Chris Christie vetoed it.

The Colorado ACLU praised the state for moving toward ending long-term solitary confinement.

“Long-term isolation costs too much, does nothing to rehabilitate prisoners, and exacerbates mental illness – or even causes it in prisoners who were healthy when they entered solitary,” spokesman John Krieger said. “Since more than 95 percent of prisoners will return to our communities, the smart approach for public safety is to focus on rehabilitation.”

The United Nations Crime Commission has backed a 15-day limit on consecutive solitary confinement and urged prisons to use shorter periods except as a last resort. In 2016, a U.N. report said 80,000 to 100,000 people are held in solitary confinement in the U.S. on any given day, and about 20 percent of prison inmates and 18 percent of jail prisoners spend time in solitary confinement over the course of a year.

Prosecutors and investigators on Friday detailed the rising tensions between the two neighbors, a combative relationship that ended with the fatal shooting of the 46-year-old Cunningham, an assistant high school principal in Aurora and former University of Colorado football star.